1. Field of the Invention
The invention relates to a water soluble thermosetting polyester resin composition and a method of preparing the same, and more particularly, to a water soluble thermosetting polyester resin composition for applying to a paint composition for undercoating a pre-coated metal (PCM) steel sheet and a method of preparing the same.
2. Description of the Related Art
Generally, a resin of paint compositions for coating a PCM steel sheet includes an acryl based resin, an urethane based resin, an epoxy based resin, a polyester based resin, a silicon based resin, a fluorine based resin, a polyvinylchloride based resin and the like. Most paint compositions are an oil type and a thermosetting type. The polyester based resin and the epoxy based resin are widely used except for a product requiring a special paint composition having specific physical properties. Among them, the paint composition employing the polyester based resin is most widely used.
The physical properties required for the paint composition for the PCM steel sheet are as follows: 1) a good mechanical property including a processibility and a pencil hardness representing the flexibility after implementing the coating process, adhesiveness to a substrate or to a topcoat layer and antifriction property, 2) a good chemical property including acid resistance, alkaline resistance, MEK rubbing property, weather resistance and the like, and 3) a good working efficiency influencing productivity and cost.
The paint composition used for undercoating the PCM steel sheet until now is mainly classified into a polyester based paint composition and an epoxy based paint composition. The polyester based paint composition for undercoating the PCM steel sheet has good processibility after finishing coating a top, good chemical resistance and good working efficiency. The polyester based paint composition, however, has a bad adhesion to the topcoat or the substrate and weak scratch resistance.
Meantime, the epoxy based paint composition for undercoating the PCM steel sheet has good adherence to the topcoat or the substrate and good scratch resistance. However, the processibility of the epoxy based paint composition is worse than that of the polyester based paint composition.
As described above, based on the different and complementary physical properties of the two kinds of paint compositions, the polyester based paint composition and the epoxy based paint composition, for undercoating the PCM steel sheet, form two separate markets for home appliances and for construction materials. Therefore, a combined type paint composition for the undercoating including the advantages of the polyester based paint composition and the epoxy based paint composition is required.
In addition, since the paint compositions are oil type, solvents added for controlling the working efficiency are aromatic or aliphatic solvents having a high boiling point, and the solvents are added excessively. Therefore, the compositions accompany an environmental contamination, and are harmful to human body. Instead of using the paint compositions including the organic solvents, the utilization of a water-dispersing type urethane resin has been preferred. However, the urethane resin has a relatively weak chemical resistance, solvent resistance, adherence, scratch resistance and thus it is inappropriate when used for coating the PCM steel sheet. Therefore, the oil type and thermosetting polyester based resin or the epoxy-based resin is regarded as the best.